1. Field of the Invention
The present invention relates to a process for continuously forming rings of thermoplastic, shrinkable tubular casings by heat treatment of the inner side of a spiraled tubular casing in the inflated state, wherein the tubular casing is filled with compressed gas of low gauge pressure section by section before running into a ring-forming tool. The invention also relates to an apparatus for carrying out this process.
2. Description of the Related Art
Sausage casings are made from straight tubular casings, which are generally single-layer or multilayer polymer casings. The starting material for these casings is biaxially oriented. By filling such sausage casings with sausage meat, straight, cylindrical sausages are obtained. In addition to such elongate sausages, ring sausages or sausage rings are also produced, starting out as straight tubular casings, which are treated in ring-forming means or ring-forming tools in order to obtain so-called ring casings. Formation of the rings is based on the fact that the tubular casings are shrinkable. One-sided heat treatment of the inner side of these tubular casings causes shrinkage, whereby the straight tubular casing is curved.
Such processes and apparatuses for producing shrinkable tubular casings in ring form by one-sided heat treatment in the inflated state have long been known. For example, DE-B 23 22 220 discloses a process in which the heat treatment of the tubular casing is performed in three phases, the inner side of the spiraled tubular casing being brought initially to a temperature below, then to a temperature above and, finally again, to a temperature below the lower limit of the shrinkage-initiating temperature range.
The apparatus for carrying out this process comprises a partially heatable, at least one-piece and substantially cylindrical roll arrangement for the spiraling of the shrinkable thermoplastic tubular casings, with a pair of pinch rolls respectively arranged upstream and downstream of the roll arrangement. Between the pairs of rolls, a gas bubble is trapped in the tubular casing. The roll arrangement has end regions for receiving the tubular casing and for delivering the tubular casing, which are kept at a temperature below the shrinkage-initiating temperature, and a region lying in between for the heat treatment of the tubular casing, which is kept at a temperature in the shrinkage-initiating temperature range for the respective tubular casings. In this case, the roll arrangement is of a multipart design, and its individual parts are connected positively but releasably to one another. The guidance for the tubular casings comprises a spiral arranged on the surface of the rolls.
DE-A 30 31 944 discloses a process and an apparatus for curving tubular casings. In the process, the tubular casing is exposed during its curving path, on the one hand, to a contact and/or radiant that, and on the other hand, to a convective heat. The heat treatment by contact or radiation is sustained longer than the treatment with convective heat. The tubular casing is filled with gas or air directly before or after the heat treatment and is wound spirally over a mandrel, the successive turns lying against one another and the contact or radiant heat acting in the space which is enclosed by the spiral turns. The apparatus has a rotatable, smooth, heat mandrel, fixing means for fastening the tubular casing to be curved on the mandrel and an apparatus for feeding the tubular casing. Together with a housing, this feeding apparatus surrounds the mandrel, includes a heating apparatus and is displaceable along the mandrel. A drive apparatus displaces the feeding apparatus in at least one direction, the speed depending on the number of turns around the mandrel and on the diameter of the tubular casing winding around the mandrel.
DE-C 23 14 767 describes a process for producing edible, ring-shaped sausage casings by extrusion blow molding and helical taking off of the inflated extruded casing by depositing it on revolving surfaces which are arranged in a continuous sequence one behind the other and run around at the delivery rate of the casing from the extruder die. If appropriate, drying, hardening or tanning and/or softening are carried out simultaneously. The casing consisting of edible material is deposited on conical revolving surfaces passing through. The apparatus for carrying out the process has a plurality of driven carrying elements, arranged one behind the other, as a deposit for the extruded, inflated casing. An adjustable lifting-off means, angled with respect to the axis of the helix, has the form of a roller for transporting a casing from one carrying element to the next. The individual carrying elements have closed conical revolving surfaces and are driven by a central, joint drive shaft. The cone of the carrying elements becomes broader in the transporting direction of the casing, and the carrying elements have projections or ribs running in the direction of the axis of the helix.
DE-B 17 04 864 relates to a process and an apparatus for continuously producing endless, spirally curved plastic tubes or casings by extruding a hot polymer mass from an annular die mounted in a blow head, by expanding the still plastic tube by means of a metered amount of air, directed into the interior of the tube, and holding back this supplied amount of air in the tube by means of pinch rolls, wherein the tube is cooled differently over its circumference. The expanding tube is asymmetrically cooled before reaching its final and greatest diameter, and the curving tube is taken in a semicircle up to the pinching point. The apparatus comprises an extruder with screw and a blow head, which is provided with an air supply and has a concentric or eccentric annular die; pinch rolls for pinching off the blown air; and means for cooling the tube differently in individual sections. Arranged behind the blow head is a nozzle ring, inclined with respect to the axis of the tube, for blowing cooling air onto the tube from all sides. The nozzle ring comprises two half-rings arranged in different planes running transversely with respect to the axis of the tube and offset by 180.degree. from each other.
WO 95/21052 discloses a process for continuously producing ring casings for sausages or hams, in which a thermoplastic cylindrical tubular casing which contains gas is wound up. A means for heat shrinkage has a multiplicity of pin-like rollers, which are arranged circumferentially and rotate about their axes, in order to spiral the tubular casing.
In the case of the known processes and apparatuses, it is necessary to prepare the tubular casing for the ring-forming process by initially winding it up on a winding drum and inflating it with compressed air. The continuous supply of compressed air is provided here from the winding drum. In this way, up to 2000 m of tubular casing can be prepared for the subsequent ring forming. The ring casing produced by the ring forming is portioned lengthwise and sectionally and is subsequently shirred, the individual ring spirals being processed. This portioning into individual ring spirals, filed with a specific air pressure, necessitates the previously described complete pre-winding and inflating of the tubular casing on the winding roller. In this case, the amount of equipment required for forming the rings is also considerable, since, for example, different regions of the ring-forming means have to be controlled such that they are at different temperatures.